A material for fuel tanks of automobiles and motorcycles is required to have not only weldability but also resistance to general corrosion on the outer side (hereinafter called "cosmetic corrosion resistance") and to corrosion caused by fuels such as gasoline on the inner side (hereinafter called "fuel corrosion resistance"). Conventionally, a ternesheet (10-25% Sn--Pb alloy-plated steel sheet) has widely been used as a material for fuel tanks. However, it has the following disadvantages: (i) Pb included in the ternesheet is harmful to the human body, (ii) the plated layer is easily dissolved in oxides of alcohols, and (iii) formation of pin holes in the plated layer is inevitable, resulting in preferential corrosion of iron from these pin holes since iron is electrochemically base compared with the plated layer, so perforation corrosion resistance is not satisfactory. An alternative to ternesheet, therefore, has long been sought.
Recently, in order to reduce the environmental problems caused by exhaust gases, an alcohol-containing fuel, called "gasohol", is being used increasingly in some countries. Gasohol is a mixture of gasoline and alcohol. For example, the mixture referred to as M15 contains about 15% of methanol, and that referred to as M85 contains about 85% of methanol. Conventional terneplate is easily corroded by such an alcoholic fuel, so a material which can exhibit improved resistance to corrosion caused by an alcohol-containing fuel is strongly desired.
For this purpose, it has been proposed to apply a Zn--Ni alloy electroplated steel sheet to fuel tanks because of its marked resistance to corrosion and its material cost. Prior art references in this respect are as follows.
Japanese Patent Application Laid-Open Specification No. 45396/1983 discloses a surface-treated steel sheet for fuel tanks having a Zn--Ni alloy plating with an Ni content of 5-50 wt % and a thickness of 0.5-20 .mu.m, and a chromate film on the Zn--Ni alloy plating.
Japanese Patent Application Laid-Open Specification No. 106058/1993 discloses a surface-treated steel sheet for fuel tanks having a Zn--Ni alloy plating with an Ni content of 8-20 wt % and a weight of 10-60 g/m.sup.2 and a chromate film on the plating.
These surface-treated steel sheets are excellent with respect to cosmetic corrosion, but they are not adequate with respect to fuel corrosion resistance. Especially, fuel corrosion easily occurs under severe corrosive circumstances, e.g., when the plates are exposed to alcohol-containing fuels contaminated with salt water. However, if a chromate film or electroplated layer is thickened so as to further strengthen protection of the tank from fuel corrosion, weldability is inevitably degraded. Weldability is an essential characteristic for materials for fuel tanks.
A dual-plated steel, such as a steel sheet having a Zn--Ni alloy plating layer and an Ni plating layer placed thereon, which is shown in Japanese Patent Application Laid-Open Specification No. 27587/1987, has also been considered, but this type of plate requires additional production steps and is costly.